This specification discloses, for the first time, a new type of electromagnetic wave which has never before been known to exist in the prior art and which results in the development of a new formation evaluation measurement, and more particularly, to a method and apparatus for receiving and measuring a newly discovered azimuthal electromagnetic wave in addition to the previously known longitudinal electromagnetic wave propagating in a formation traversed by a borehole.
Electromagnetic (EM) wave propagation tools (EPT) are used for investigating a formation traversed by a borehole and usually comprise one or more transmitters for transmitting electromagnetic energy into the formation and one or more receivers for sensing a signal propagating in the formation corresponding to the transmitted electromagnetic energy. The electromagnetic energy has been known to include longitudinally propagating electromagnetic waves. Prior art theoretical models of electromagnetic wave propagation tools in boreholes at microwave frequencies have assumed that the transmitting and receiving antennas are situated on an infinite metallic (flat) ground plane, and not on a curved plane. See, for example, U.S. Pat. No. 4,704,581 to Clark which appears to have made this same assumption which was made in connection with the prior art theoretical models. The physics of this assumption is based on the fact that the wavelengths of the EPT radiation in most rock formations of interest are small compared to the radius of curvature of the EPT antenna pad 0.1016 m). Therefore, since the transmitting and receiving antennas were assumed to be situated on an infinite flat plane, the prior art theoretical models did not lend themselves to the study and/or discovery of other than longitudinally propagating electromagnetic waves. In addition, solving the boundary value problems that must be confronted to theoretically investigate transmitter and receiver antenna pad curvature effects is considerably more complex than for planar (flat) geometry problems. Indeed, for planar layered media situated on an infinite metallic (flat) ground plane having dipole sources, solutions to the EM boundary value problems can be expressed in terms of Sommerfeld integrals and can be found in many textbooks on the subject. However, a search of the well logging and EM literature failed to locate published solutions to the two boundary value problems in this specification, which involve a curved antenna and receiver pad. Since the prior art models assumed that the transmitting and receiving antennas were situated on an infinite, flat metallic ground plane, there has been little impetus in the prior art to study any electromagnetic waves generated by EM transmitters other than longitudinally propagating waves. Therefore, the EPT tools of the prior art disclose transmitters which transmit and receivers which receive longitudinally propagating electromagnetic waves only.